Sound attenuator for air impellers



Aug. 12, 1941. E. H. HARRIS 2,252,256

SOUND ATTENUATOR FOR AIR IMPELLERS Filed .Jan. 11, 1939 2 Sheets-Sheet lINVENTOR, I

1941- H. HARRIS 2,252,256

SOUND ATTENUATOR FOR AIR IMPELLERS Patented Aug. 12, 1941 UNITED STATESPATENT OFFICE SOUND ATTENUATOR FOR AIR IMPELLERS Eliot HuntingtonHarris, New York, N. Y.

Application January 11, 1939, Serial No. 250,365

3 Claims.

My invention relates to a device for attenuating the sounds incident tothe rapid rotation of impellers in air.

The device constituting the subject matter of my invention althoughdesigned primarily for use in connection with the standard types ofimpellers or propellers utilized in aeronautics is adaptable forVentilating fans or for special types of impellers or propellers suchfor instance as those of the enclosed blade and slotted disc types.

My invention, briefly stated, resides in the disposition about theradial blades of an air or gas impeller or propeller, in circumferentialrelationship, of an annuloid enclosure, the inner boundaries of whichwill be so disposed with respect to the rotating impeller blades as toprescribe or define a zone or port for intake and discharge of the air,partaking of the nature of a nozzle, and which for the sake of brevitywill be hereinafter referred to as a nozzle.

The said annuloid enclosure comprises two or more chambers with accessto the space circumscribed by the nozzle through an aperture or seriesof apertures through which may pass the gas or air, as well as the soundWaves created by the action of the impeller. These chambers will havesuch construction and relation one to the other as will serve toattenuate those sounds whose amplitude it is desirable to reduce.

The primary object of my invention is to reduce the volume of thosedominant sounds created by the action of a rotating impeller in a fieldof gas or air, thereby reducing the facility with which the presence ofan airplane may be detected, and also to diminish the noise created asstated, within an airplane.

By placing a nozzle around an air impeller it is obvious that bothsuction and discharge coefficients will be improved. .I have discoveredthat by use of a properly designed nozzle, with all other conditionsmaintained, the quantity of air displaced will be increased, or, as acorollary, the thrust of the impeller will be substantially increased toan extent sullicient to compensate for the additional weight when thenozzle is applied to an aircraft. While the employment merely of anozzle will eliminate certain sounds resulting from the sudden andimpulsive change of direction in the air sucked into the impellerstream,

the diminution thus obtained in the volume of twice the magnitude of oneof these sounds, it will be seen that the elimination of a sound ofminor magnitude will result in only a slight reduction of the totalvolume of sound. It should be further explained that studies of thesound produced by a fast moving propeller in air show that the soundconsists of a series of sounds of independent frequencies as well asmany harmonies of the fundamentals. Thus frequencies from approximatelyto 12,000 per second have been located when examining a propeller from anear-by location. Examined from a distance where no reflecting objects,such as ground contour, may interfere, there will be found certaindominant frequencies, F, that, in most instances, are a function of thenumber of blades in the propeller and of the speed of its rotation, theflexure vibrations which may be computed from the physical design of theblades and shaft, as well as those indefinite frequencies associatedwith the shedding of eddies from the blades. My device will attenuatethese dominant sounds.

In the accompanying drawings, illustrating different embodiments of myinvention:

Fig. 1 shows, in front elevation, a nozzle circumscribed about arotating impeller with part of the inner, directing surface removed tobetter show the inner chambers and particularly the radial walls,embodying my invention;

Fig. 2 is a cross section taken on the line 2-2 of Fig. 1 and shows thecircumferential annuloid enclosure as being divided at any cross sectioninto one impedance chamber and one resonator chamber, the direction ofair flow being indicated by the series of arrows, the air approachingthe device at its leading edge and passing beyond the device at itstrailing edge;

Fig. 3 shows an embodiment of my invention wherein at any cross sectionare employed two impedance chambers and two resonating chambers within anozzle placed circumferentially about a rotating impeller;

Fig. 4 shows a propeller with a circumferential nozzle, and at the crosssection illustrated, one impedance and two resonating chambers, thenozzle face forming one enclosing wall.

Proceeding now to a more detailed description of my invention andreferring to Fig. 1 and Fig. 2 of the drawings, the numeral l denotes anozzle provided with an interior directing surface shown in crosssection at 2, shown as partly cut away in Fig. 1, which is placed aboutthe periphery of the circle described by the propeller 3 in rotation onthe shaft 4. In the cross sectional view of the nozzle Fig. 2, there isshown an outer enclosing imperforate wall of the nozzle which isindicated by the numeral 5. The annuloid created by the enclosingsurfaces 2 and has its axis coincident with the axis of the propellershaft 3. Within the annuloid there is shown a dividing wall 15 whichseparates the annuloid enclosure into the impedance chamber 6 andresonator chamber "a. In the directing surface, indicated by the numeral2, there are provided a series of orifices 8, giving access to thechamber 5. In the dividing wall Ill there are orifices 9 giving accessfrom the impedance chamber 5 to the resonance chamber '1. The annuloidchambers 6 and 'I' may be increased in frequency by placing one or morelimiting walls, indicated by the reference numeral l2 as shown, saidwalls l2 being approximately radial to the axis of the shaft 4 andinside the annuloid space enclosed by the walls 2 and 5.

In Figure 3 there is illustrated a nozzle which is designated by thereference numeral l which departs in the shape of the directing surfacefrom that of the nozzle illustrated by Figs. 1 and 2, and in that theannuloid space between the walls 2 and 5' has been further divided by asolid wall H and there are provided two sets of impedance chambers 6 andtwo sets of resonator chambers I. The orifices 8 provided in the wall 2'of the nozzle l are here arranged in two groups, one on the suction sideof the plane of propeller rotation and one on the discharge sidethereof. There are also two sets of orifices 9 interconnecting theimpedance chambers 6' and resonating chambers I.

Fig. 4 is similar to the construction of Fig. 3, except that the singleimpedance chamber 6 has access through two sets of orifices 9 to the tworesonator chambers l" and only one series of apertures givecommunication from impedance chamber 5" to the source of noise.

The proper design of my impedance and resonator chambers is dependentupon a large number of variables. The calculus employed may easilybecome too involved for ready solution and there are also certainconstants required which must be empirically determined. 1 havetherefore developed a series of ratios for the design which may beemployed with good results.

It must be understood that for each change of impeller design, diameterand critical speed, a different design of my device will be required.There is, however, a considerable tolerance available in the design ofthe chambers due to the fact that impedance and resonance will beeffective over a fairly wide range of frequencies in any given orificeor chamber, and furthermore the nonharmonic relation will maintainbetween impedance and resonator chambers over any small range of volumeor shape change in such chambers. Furthermore the attenuating effectresulting from my proposed arrangement of chambers and their associatedorifices is active over the full range of harmonics and to a slightlylesser extent to the 5/4, 2/3 series of frequencies.

The arrangement of chamber for my invention as illustrated by Fig. 3 ofthe drawings will be described below. The methods for calculation ofother types will be obvious to one skilled in the art.

Impedance of orifice leading from a reservoir is given by E. J. Irons inthe familiar formula:

where Z=impedance; azvelocity of sound in medium; =density of medium;V=volume of chamber; F=frequency; c=conductance.

From this it will be seen that the impedance varies inversely with thevolume and directly with the conductance. Therefore the impedancechamber should be smaller than the resonator chamber and by calculationit may be estimated that the best results are obtained with ratios from3.6:1 to 7.6:1. Although attenuators have been made with ratios of 1.621there were special features which would not necessarily hold for thisdesign. The actual size and shape must be on an approximate ratio oftuned frequencies as will be later explained.

An approximation may now be made of the natural frequency of theresonator chamber by the standard formula:

(1 C 21r V where the symbols are the same as above. Multiply theresulting frequency f by 1.15 to get an approximation of the actualfrequency for the chambers indicated by the drawings. The result of thiscomputation may be compared with the dominant frequency F to beattenuated. If the estimated frequency is too low, baflies may bedesigned to be placed approximately radially to the axis of the impellerand the volume of the chamber reduced, then another estimate made.

Another method of computing the factors involved is to consider oneimpedance chamber and its connecting resonator chamber as a coupledsystem. Then the natural frequency of the coupled system where M=thecombined vibrating mass, mi -the mass within the impedance chamber andma: the mass within the resonator chamber. The mass may be estimated byM:=A p Z, where A: area, =density, l=length including end correction.End correction would be approximately 0.29 times the diameter ofequivalent cylinder. The density must be chosen for the atmosphericconditions considered. At one atmosphere pressure, 0 C. and no humidity,p may be assumed as 0.00129 in the C. G. S. system. The naturalfrequency of the resonator chamber 712, and the impedance chamber 121may be computed by the formula;

l ing m where L, the inertance,

and C, the capacitance,

It may also be noted that for a thin plate opening the value of A1 takeson the character of the radius as the inertance is considered asoccurring in the orifice whereas A in the capacitance formula is afunction of the volume and should be so computed. Integrals of theseformulae may be used when the shape of the chambers is complex, as, forexample, when a complete annuloid space is used as a single chamber. Itwill be found, however, that for all usual conditions the chambers willconsist of an annuloid space divided by radially disposed walls.

When the frequency of the resonator chamber is estimated approximatelyto the frequency F, or 2 F, using the lowest value of the exponentpossible, the pitch of one resonator chamber should be empirically tunedto the frequency F, or to a frequency of the series noted above. Whenthe above conditions are satisfied the correct design has been attained.

The frequency of the impedance chamber will then be attained as follows:first, determine the logarithm of the frequency of the resonatorchamber; second, find the logarithm of twice that number; then, to thelogarithm-of the first frequency add one twelfth of the differencebetween the two logarithms found. The antilog of this will be the basefrequency of the impedance chamber, A frequency of 1/2 times this figurewill be used to determine the approximate volume by the formula andtested until the volume of the impedance chamber falls within the ratiofigures given previously. This may be done by use of the resonanceformula given above to obtain the approximate volume when the exactdesign will be obtained empirically.

The ratio of the total area of openings, apertures, slots or means ofcommunication from the impedance chamber to the cross sectional area ofthe nozzle at the vena contracta may be approximately 2:1.

It should be kept in mind that when testing the natural frequency of achamber that the openings in the chamber wall should be according totheir final location as these openings afiect the frequency.

It is to be understood that my invention is not to be regarded as beinglimited by any or all of the constructions herein illustrated anddescribed, such illustrations and descriptions being merely for thepurpose of showing how the principles underlying my invention may becarried into practise.

The particular curvature of the intake and discharge nozzle is no partof my invention. From some point either side of the plane of propellerrotation the inner boundaries of the annuloid are divergent from thepropeller axis.

The materials employed in the construction may be any lightweightmaterial lending itself to thin wall construction, such as steel oraluminum sheet or compressed asbestos board or the like. Welded steelsheets have performed satisfactorily in tests.

The support for my device may be a simple frame attached to thestructure supporting the prime mover, or my Whole device may be attachedto the impeller itself, all as will be understood by those skilled inthe art without further disclosure. If the latter construction isemployed, the lift of the rotating cylinder must, of course, be balancedand the yaw engendered thereby must be corrected by proper vanes. Soundinsulated walls and other construction details are of course applicablebut form no part of my invention. Neither does the shape or design ofthe impeller become any part of this invention.

I claim:

1. A sound attenuator for air impellers comprising an annuloid having anouter imperforate surface and an inner perforated surface, said surfacesjoined together at the leading and trailing edges, said inner surfacehaving the greater diameters of its annular form at its axialextremities, the lesser diameter occurring in the central section of itsaxial length, walls Within said annuloid to divide same into impedanceand resonance chambers, means for passage of sound Waves from exteriorof said annuloid into said impedance chambers, and orifices between saidimpedance chambers and said resonator charn bers.

2. An attenuator for the sounds created by the rapid rotation of apropeller on its shaft, said attenuator comprising an annuloid whoseaxis coincides with the axis of said shaft, said annuloid being formedby an outer imperforate surface and an inner surface containingopenings, the axial extremities of said inner surface of the annuloidhaving greater diameter than the central section of said inner surface,walls between the inner and outer surfaces of said annuloid dividing theenclosure into a multiplicity of chamber groups, each group beingcomposed of one impedance chamber and one associated resonator chamberand apertures in said inner surface for the passage of sound waves fromthe exterior of said annuloid into said impedance chambers.

3. A sonic attenuator for air impellers compirsing an annuloid formed byan outer imperforate and an inner perforate surface, said inner surfacebeing divergent from its central section to its leading edge, said innersurface being divergent from its central section to its trailing edge,walls within said annuloid from and between said inner and outersurfaces dividing said annuloidal enclosure into a multiplicity ofimpedance and resonator chambers, means of communication between eachimpedance chamber and the exterior of the annuloid through said innerperforate surface and orifices between said impedance chambers and theiradjoining resonator surfaces.

ELIGT HUNTINGTON HARRIS.

